1. Field of the Invention
The present invention relites generally to improvements in antenna technology, and particularly to advantageous aspects of an antenna coupler for use with portable telephones.
2. Description of the Prior Art
Portable handheld wireless communications devices, especially telephones, have increased dramatically in popularity over the past several years. Because of the proliferation of new wireless services into various frequency bands, including previously unused frequency bands, there has developed a need for a portable telephone operating in more than one frequency band. For example, a portable telephone may be called to operate in both the cellular frequency band (824-896 MHz) and the personal communications system (PCS) band (1850-1990 MHz).
When the portable telephone is used inside an automobile, the signal level is significantly reduced due to shielding effects of the car body. Even the presence of other occupants within the vehicle may contribute to the shielding. In order to mitigate these effects, the portable telephone is typically coupled to an antenna external to the vehicle. In one system, an external antenna coupling unit kit is installed in the interior of the vehicle. The kit includes a shell that has a cavity that is shaped to receive the portable telephone unit, and a coaxial cable connection to an antenna mounted to the exterior of the vehicle.
Typical prior-art devices provide electromagnetic coupling between the telephone's built-in antenna and the vehicle's external antenna. This generally results in high transmission loss (weak coupling), which in turn makes it necessary to use additional bi-directional amplifiers for loss compensation. Further, prior designs do not typically provide for a dual band operation of the device.
Consequently, there is a need for a low-loss antenna coupler transferring the signal to and from the external antenna in at least two frequency bands.